The present invention relates to a method for checking the wall thickness of a layer, wherein the wall thickness is compared at various points of the layer by reference to the distance between a free surface of the layer and a reference member contiguous with an opposite surface of the layer to ascertain maxima and minima of the relative thickness values.
A known method of the above type is based on detecting maxima and minima of the wall thickness, storing them and indicating maximum and/or minimum values, and possibly also setting off an alarm if limiting values are exceeded. The wall-thickness measurement is effected by means of inductive transmitters. An absolute measurement is made difficult as a result of the dependence of the measurement on the temperature and other influences. It is true that it is known to remove the measuring head or heads periodically from the object to be measured, for example a cable or wire, in order to make a zero adjustment. This procedure is complicated, however, and in some circumstances, important measured values are lost during such adjustment.
It is also known to detect the layer thickness at four points each offset by 90.degree. and to draw conclusions about the eccentricity from the measured values. It is further possible to determine the local layer thicknesses from the eccentricity as well as from the diameters of the layer and of the coated core from an angular function (U.S. Pat. No. 3,426,437). In this case, however, no gain in measuring accuracy is aimed at or achieved.